Private voice networks include telephone apparatus, telephone exchanges (PABX) and transmission equipment. State of the art transmission equipment utilizes compression multiplexers, which convert analog voice signals to digital signals at low bit-rates of 9.6 kbps or even lower. Various compression methods are employed which all take advantage of the inherent redundancy which exists in human speech.
If compressing multiplexers are installed between PABX's, as shown in Fig. BA-1, the voice signal may be compressed and decompressed more than once in PABX networks with more than one hop. Each compression/decompression pair degrades the voice signal quality somewhat, and therefore in large networks it is very likely that the end-to-end voice signal quality will be unacceptable to the users.
To overcome this problem, more complex multiplexers have been developed, as shown in Fig. BA-2, in which the multiplexers also handle the routing function. Each voice channel is compressed by the first multiplexer and decompressed by the last multiplexer in the chain of multiplexers. All other interim multiplexers bypass the digital voice signal, such that voice signal quality is kept at a constant level. The drawback of this approach is that since the compressing multiplexers perform the routing function, they are relatively complex and expensive.
In the present specification, the first (source) and last (destination) nodes in a chain of nodes handling a call are termed "pass through" nodes, whereas the intermediate nodes, i.e. all nodes handling the call other than the first and last nodes, are termed "bypass" nodes.
There exists PBX/PABX equipment on the market that includes routing functions. As a result, a network can be formed among the PBX/PABX using regular voice channels. If the voice channel bandwidths are compressed, a routing multiplexer is required in order to avoid multiple voice compression/decompression along some of the network routes.
State of the art telecommunications systems include the following: The Integrated Digital Network Exchange, ADNX/60 and IDNX/20-S systems, all marketed by Network Equipment Technologies, and the 3630-MainStreet Primary Rate Multiplexer, marketed by Newbridge.